1. Field of the Invention
The present invention relates to an intersubband light emitting element such as a light emitting diode and a laser diode. Such an element emits light of which a wavelength ranges from a near infrared region to a terahertz region.
2. Description of the Related Art
The intersubband light emitting element is attractive as a light source for emitting light having a wavelength that ranges from the near infrared region to the terahertz (THz) region, because it has a relatively high oscillator strength and is capable of controlling a transition wavelength of the light widely utilizing the structure of the element. A conventional intersubband light emitting element has a heterojunction structure referred as a type-I structure, such as GaAs/AlGaAs and GaInAs/AlInAs.
However, the element of the type-I structure has a disadvantage that the amount of carrier leakage is high and thus the injection efficiency of the carriers is low when the carriers are injected into a quantum well of the element. This is one of the reasons why, under existing circumstances, the threshold current density of the element increases during the laser oscillation, and makes it difficult to oscillate the element continuously under a room temperature condition.
It is an object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current.
It is another object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current, that is capable of emitting light having a wavelength of the THz region.
It is another object of the present invention to provide an intersubband light emitting element with a minimized leakage current and a decreased threshold current, that is capable of emitting light having a plurality of predetermined wavelengths such as blue light and red light at the same time.
According to one aspect of the present invention, there is provided an intersubband light emitting element comprising:
a semiconducting substrate;
a first layer composed of a first semiconducting material;
a second layer composed of a second semiconducting material and making a heterojunction with the first layer, the top of a valence band of the second semiconducting material being higher in energy than the bottom of a conduction band of the first semiconducting material; and
a third layer making a heterojunction with one of the first and second layer and having a superlattice structure,
wherein one of the first and second layer is provided on the semiconducting substrate directly or through at least one semiconducting layer.
Intersubband light emission occurs when carriers injected into the second subband within a quantum well of the first layer are relaxed to the first subband within the same quantum well. In order that the intersubband light emission takes place effectively, it is necessary to prevent leakage of the injected carrier and thereby utilize the intersubband transitions of the entire carriers.
With the above-mentioned element according to the invention, the first layer and the second layer form a broken gap structure, and this structure prevents leakage of the carriers or electrons, with the result that the threshold current of the element is decreased.
The third layer having the superlattice structure forms a new energy gap. Consequently, the leakage current is further reduced and the threshold current is also further decreased.
The element according to the present invention may be constituted such that the functions of the first and the second layers are reversed to each other. In this instance, the intersubband light emission occurs when carriers injected into the second subband within a quantum well of the second layer are relaxed to the first subband within the same quantum well. The carriers are then pulled from the first subband rapidly due to an interband tunneling effect of the heterojunction structure formed with the first layer and the second layer. Similarly, the broken gap structure prevents the leakage of the carriers or holes, and the leakage current is further reduced because the third layer having the superlattice structure makes a heterojunction with the first layer.
Preferably, the element comprises a plurality of sets of the first and second layers, the sets each having a quantum well width of one of the first and second layers, the quantum well widths being different from each other among the sets. The wavelength of the light emitted from the element or a light emitting portion formed with the quantum well becomes longer as the well width becomes wider, and it is thus possible to emit the light having a plurality of predetermined wavelengths such as blue light and red light at the same time.
By way of a concrete example, the first semiconducting material is one of InAs and InAs with an impurity, the second semiconducting material is one of GaSb and GaSb with an impurity, and the superlattice structure is one of an InAs/AlSb superlattice structure, an InAs/AlSb superlattice structure with an impurity, a GaSb/AlGaSb superlattice structure, a GaSb/AlGaSb superlattice structure with an impurity, a GaSb/AlSb superlattice structure, and a GaSb/AlSb superlattice structure with an impurity. The impurity of the InAs is one of Al and Ga instead of one of In and Si, one of Sb and P instead of As and so on. The impurity of GaSb is one of In and Al instead of one of Ga, Si and Be, one of As and P instead of Sb.
According to another aspect of the present invention, there is provided an intersubband light emitting element comprising:
a semiconducting substrate;
a first layer composed of a first semiconducting material;
a second layer composed of a second semiconducting material and making a heterojunction with the first layer, the bottom of a conduction band of the second semiconducting material being higher in energy than that of the first semiconducting material or the top of a valence band of the second semiconducting material being a lower in energy than that of the first semiconducting material;
a third layer composed of a third semiconducting material which is same as the first semiconducting material and making a heterojunction with the first layer; and
a forth layer composed of a forth semiconducting material and making a heterojunction with the third layer, the top of a valence band of the forth semiconducting material being a higher in energy than the bottom of a conduction band of the second semiconducting material or the bottom of a conduction band of the forth semiconducting material being a lower in energy than the top of a valence band of the second semiconducting material;
wherein one of the first and forth layer is provided on the semiconducting substrate directly or through at least one semiconducting layer.
When widening the well width of the quantum well to prolong the wavelength of the emitting light to the THz region, the second subband within the quantum well becomes lower than the top of a valence band of the third layer. Therefore, the carriers injected into the second subband are not blocked by the barrier and thus the injection efficiency of the carriers decreases.
With the above-mentioned element according to the invention, the quantum well is formed with the first to third layers. In this case, in order to prevent the decrease of the injection efficiency due to the prolongation of the wavelength to the THz region, a projecting potential barrier is formed at the center of the quantum well by the second layer of which the bottom of a conduction band is higher in energy than that of the first layer. Such a projecting potential barrier has an effect on only the carrier distributed around the center of the quantum well. Accordingly, it is possible to increase the energy of the first subband without changing the energy of the second subband. Thereby, it is possible to decrease an energy space of the subband without affecting the injection efficiency of the carriers. As a result, it is possible to realize the element with a high efficiency even if the light having the wavelength of the THz region is emitted.
The element according to the present invention may be constituted such that functions of the first and the third layers are reversed to each other. In this instance, the intersubband light emission occurs when carriers injected into second subband within a quantum well of the second layer are relaxed to first subband within the same quantum well.
Preferably, the element comprises a plurality of sets of the first to forth layers, the sets each having a potential barrier of the second layer, heights of the potential barriers being different from each other among the sets. In this way, the element capable of emitting the light with a plurality of wavelengths can be achieved. The reason is that the injection into the second subband can be performed with the same efficiency because the energy of the second subband does not substantially change even if the height of the potential barrier changes, and no carrier always exists at the first subband because the interband tunnel effect of the heterojunction structure formed with the first and second layers is remarkable and thus reabsorption of the carriers hardly occurs.
By way of a concrete example, the first semiconducting material is one of InAs and InAs with an impurity, the second semiconducting material is one of AlInAs, AlInAs with an impurity, GaInAs, GaInAs with an impurity, InAsSb, InAsSb with an impurity, AlGaSb and AlGaSb with an impurity, the third semiconducting material is one of GaSb and GaSb with an impurity.
By further comprising a fifth layer with a superlattice structure, a new energy gap is formed. As a result, the leakage current is further reduced and the threshold current is further decreased. The superlattice structure is one of an InAs/AlSb superlattice structure, an InAs/AlSb superlattice structure with an impurity, a GaSb/AlGaSb superlattice structure, a GaSb/AlGaSb superlattice structure with an impurity, a GaSb/AlSb superlattice structure, and a GaSb/AlSb superlattice structure with an impurity.